1. Field of the Invention
This invention relates to an electrical position sensing circuit and particularly a liquid level sensing circuit used to make possible a non-contact electronic fuel level sensor for measuring the amount of fuel remaining in a motor vehicle gas tank.
2. Background Art
Fuel level sensors known for monitoring the level of fuel within a gas tank are resistive, ultrasonic, electromagnetic, and shaped electromagnetic field in nature. Resistive type sensors rely on contact connections to a resistive element. A float and a pivot arm move such that when the fuel tank is full, the wiper contact is at one extreme of the resistive element, and when the tank is empty the wiper contact is at the other extreme. It is known that such contact type sensors wear over time, making non-contact type sensors desirable. Ultrasonic, electromagnetic, and shaped electromagnetic field sensors have complex electronics to drive the sensor element and determine the fuel level. Their complexity makes component costs high.
Fuel level sensors are particularly well known in the automotive industry where they are used extensively in passenger cars and trucks. For many years automotive manufactures have relied on the resistor card technology to provide a feasible and economical fuel level sensing approach for use in motor vehicles.
The resistor card can be characterized as an electrical potentiometer configured as a variable resistor. Its wiper is mechanically fixed to a float or combination float and float arm mechanism that rises and falls with the level of liquid in a fuel tank. The value of resistance developed across the potentiometer's resistor element as a result of wiper positioning, corresponds to the position of the float, hence the level of liquid in the fuel tank. This resistance value is then read by configuring the sensor resistor element in series with a second known resistance. When a known voltage is applied across the two series resistive elements a voltage divider circuit is created wherein the voltage produced at the common electrical node of the two resistive elements directly correlates to the position of the potentiometer's wiper and hence the position of the attached float.
Despite its moderate precision and performance, the resistor card has been the preferred fuel level sensor supplied to the automotive industry. Only recently have automotive manufacturers begun to search for more robust and precise technologies that can replace the resistor card as a fuel level sensor.
Manufacturers trying to reduce product warranty costs and offer products with longer life must look beyond resistor card technology. Although regarded as low cost, the resistor card is not a cost effective solution for fuel level sensing when its characteristics of product reliability and service life are considered. The major fault of the resistor card is its reliance on a mechanical wiper to maintain electrical contact to the resistive element within its assembly. Level sensors that incorporate a mechanical contact have a number of well-known problems. The mechanical movements within any mechanism make those components susceptible to wear, fatigue, and loosening. This is a progressive problem that occurs with use and leads to eventual failure after a sufficient amount of movement has occurred.
In fuel level sensing, a source of mechanical failure for the resistor card comes from fluid sloshing inside the fuel tank thus creating unwanted twisting and levering of the float and float arm. This force is transferred from the float to the wiper of the resistor card where it stresses the components by applying alternate weak and strong contact force between the wiper and the resistive element. Under these conditions the wiper may eventually fatigue losing its spring characteristics and may even loosen from its rotational pivot hub. Eventually the wiper will lose electrical contact with the resistor element.
Exposure to continuous vibration, like that experienced on a moving vehicle, produces excessive movement of the wiper with respect to the resistor element. While stationary about one position, dithering of the wiper against the resistor element leads to excessive wear and pitting of the resistor element at that particular wiper position. This action eventually damages the resistor element by altering the resistance of the sensor at that position.
Over time contaminants build up on the wiper and resistive element. This contamination can produce poor electrical contact between the wiper and the resistive element leading to sensor inaccuracies or malfunction. Increasing the force of the wiper against the resistive element will help to keep the contact point between the wiper and resistive element clean thus reducing possible failures. However, the additional force applied between the wiper and resistive element increases the frictional force against each other. This accelerates wear on the components thereby reducing the sensor's service life.
Another problem experienced with the resistor card is its inability to survive long-term exposure to newer types of fuels now used in automobiles. Exposure to Methanol, Ethanol, peroxide and other fuel additives are known to breakdown the inking adhesives used in making the resistor element of the card. Eventually contact of the wiper and the resistive element opens causing operational failure of the level sensor.
One solution to overcoming the inherent problems of mechanical type sensors like the resistor card is found in non-contact type sensors. In recent years magnetic flux sensor technology, particularly that of the Hall effect sensor, has developed into a robust and reliable technology.
The integration of custom circuitry with the Hall effect sensing element makes it possible to produce Hall effect sensors that have advanced features like analog, PWM, and digital output capability. Compensation circuitry now available in Hall effect sensors allow for much greater accuracy and linearity over operating temperatures. Hall effect sensors now offer programmable features that permit adjustable control of output signal ratiometry, sensitivity, voltage offset, temperature coefficient, and output signal range limiting. Even advanced features of functional diagnostics can be integrated into the Hall effect circuitry.
An analog Hall effect sensor is designed to output a voltage that is proportional to the strength of a magnetic field of which it is exposed to. In truth the Hall effect sensor responds to the strength and polarity of the magnetic field that passes perpendicular to its internal magnetic flux sensing element. That is to say that the Hall effect sensor will produce the greatest change in output voltage when the magnetic lines of flux that make up the magnetic field are perpendicular to the magnetic flux sensing element, and have no response when the lines of magnetic flux are parallel to the magnetic flux sensing element. The physical relationship between the magnetic field and the Hall effect sensor can be altered by moving the magnetic field with respect to a stationary Hall effect sensor.
To produce a change in the output of the Hall effect sensor, the relative air gap between the magnet and Hall effect sensor can be changed. As the magnet moves further from the Hall effect sensor, the sensor is exposed to lesser magnetic field thereby inducing less effect on the sensor. Likewise as the magnet is moved closer to the Hall effect sensor, the sensor is exposed to greater magnetic field that produces more change of the sensors output voltage. This functional; principle is demonstrated by use of a rotating involuted magnet in Nartron U.S. Pat. No. 6,396,259, Electronic Throttle Control Position Sensor.
Another useful approach to varying the magnetic field strength, of which the Hall effect sensor is exposed to, is to use a tapered magnet. A magnet's field strength increases in proportion to its thickness. Increasing the thickness of a magnet placed before the Hall effect sensor will increase the output response from the sensor. This principle is taught in CTS U.S. Pat. No. 6,211,668, Magnetic, Position Sensing Having Opposed Tapered Magnets.
Yet another approach places a pole piece behind the Hall effect sensor. In this configuration the Hall effect sensor is positioned between the magnet and pole piece. As the pole piece moves towards the Hall effect sensor, the air gap between the pole piece and Hall effect sensor is reduced. Likewise the distance gap between the pole piece and the magnet is reduced, thereby attracting the magnetic field towards the pole piece and passing more magnetic lines of flux through the magnetic flux sensing element and into the pole piece. This functional principle is demonstrated in Nartron U.S. Pat. No. 6,396,259, Electronic Throttle Control Position Sensor, using an involuted pole piece that is rotated about its center axis to produce a closing air gap with rotation.
Since the magnetic flux sensing element is sensitive only to lines of magnetic flux passing perpendicular to its element plate, and therefore does not respond to magnetic flux that is in parallel with its element plate, a third preferred method of producing a response in the Hall effect sensor exists. Rotating the magnet about the Hall effect sensor changes the angle in which the lines of magnetic flux pass through the magnetic flux sensing element. The resultant magnetic field acting perpendicular to the magnetic flux sensing element generates a response from the sensor proportional to this perpendicular field strength. This approach is described in Nartron U.S. Pat. No. 6,396,259, Electronic Throttle Control Position Sensor.
Two additional prior art patents owned by the assignee of the invention relating to similar technology are U.S. Pat. No. 5,504,427 to Cooper et al and U.S. Pat. No. 5,150,615 to Rymut et al. These patents are incorporated herein by reference.